1. Field of the Invention
The invention relates to resin extrusion and particularly to extrusion heads and methods for extrusion of single and multi-layer resin parisons, and an intermediate product and products formed from the parison.
2. Brief Description of the Prior Art
Commercially available thermoplastic resins are extruded at temperatures specific to the particular resin. In order to maintain the properties of the resin, the flow temperature during extrusion must be within a pre-determined temperature range for the resin. The properties of the resin may be degraded when the resin is heated appreciably above its flow temperature.
Prior extrusion heads are incapable of forming a parison having a layer of undegraded low flow temperature resin adjacent a layer of high flow temperature resin where the properties of the low temperature resin would be degraded if the resin were heated to the flow temperature of the high temperature resin. Heat supplied to the head by the high temperature resin raises the temperature of the flow path of the low temperature resin and heats this resin sufficiently to degrade its properties. This limitation in conventional extrusion heads has prevented the manufacture of parisons with adjacent undegraded layers of high and low temperature thermoplastic resin and the manufacture of blow molded and other types of articles from such parisons.
Parisons may be extruded with a barrier resin layer and tie resin layers on either side of the barrier layer for bonding the barrier layer to the surrounding support layers. The barriers resin is considerably more expensive than the support layer resins which, for example, may be polyolefins or polycarbonate. Because of the cost of the tie and barrier layers, the industry has attempted to reduce the thickness of these layers. Successful reduction of layer thickness requires accurate control over the extrusion process to assure each thin layer is of uniform thickness and continuous, that is, free of holes. Holes in a tie layer prevent desired adhesion between the barrier layer and the adjacent structural layer. Holes in the barrier layer destroy the barrier properties of the product formed from the parison. Extrusion of these very thin layers is made difficult by the necessity of eliminating knit lines and preventing heat degradation of the low temperature resins while in the extrusion head.
Co-extrusion heads receive heated, softened and plasticized resins from individual extruders through inlet ports, flow the resin on to the mandrel and then on the mandrel through an annular mouth. Successive layers are flowed onto the mandrel. A multi-layer parison is extruded out of the mouth of an extrusion die. The die may be modulated to vary the thickness of the parison as required to assure that the blow molded product has a uniform wall thickness relative to blow ratios.
Conventional co-extrusion heads define flow paths for each resin in an integral assembled head. The positions of the flow paths in the head are fixed and cannot be changed. Each flow path must be used at its intended location within the head. Conventional co-extrusion heads flow parison layers on cylindrical, constant diameter mandrels.
In extrusion cross head dies it is conventional to flow the resin first around the mandrel and into an equilibration chamber spaced from the mandrel. Resin flows from this chamber along a generally conical transfer passage to a mouth located axially downstream from the chamber and opening into an extrusion channel at the mandrel. The cross sectional flow area of the transfer passage decreases from the chamber to the mouth, thereby increasing resistance to the flow of the resin and undesirably increasing the temperature of the resin.